Azatruxene-Based, Dumbbell-Shaped, Donor–π-Bridge–Donor Hole-Transporting Materials for Perovskite Solar Cells

Article abstract of DOI:10.1002/chem.202002115

Three novel donor–π-bridge–donor (D-π-D) hole-transporting materials (HTMs) featuring triazatruxene electron-donating units bridged by different 3,4-ethylenedioxythiophene (EDOT) π-conjugated linkers have been synthesized, characterized, and implemented in mesoporous perovskite solar cells (PSCs). The optoelectronic properties of the new dumbbell-shaped derivatives (DTTXs) are highly influenced by the chemical structure of the EDOT-based linker. Red-shifted absorption and emission and a stronger donor ability were observed in passing from DTTX-1 to DTTX-2 due to the extended π-conjugation. DTTX-3 featured an intramolecular charge transfer between the external triazatruxene units and the azomethine–EDOT central scaffold, resulting in a more pronounced redshift. The three new derivatives have been tested in combination with the state-of-the-art triple-cation perovskite [(FAPbI₃)_0.87(MAPbBr₃)_0.13]_0.92[CsPbI₃]_0.08 in standard mesoporous PSCs. Remarkable power conversion efficiencies of 17.48 percent and 18.30 percent were measured for DTTX-1 and DTTX-2, respectively, close to that measured for the benchmarking HTM spiro-OMeTAD (18.92 percent), under 100 mA cm^?2 AM 1.5G solar illumination. PSCs with DTTX-3 reached a PCE value of 12.68 percent, which is attributed to the poorer film formation in comparison to DTTX-1 and DTTX-2. These PCE values are in perfect agreement with the conductivity and hole mobility values determined for the new compounds and spiro-OMeTAD. Steady-state photoluminescence further confirmed the potential of DTTX-1 and DTTX-2 for hole-transport applications as an alternative to spiro-OMeTAD.

Full text of DOI:10.1002/chem.202002115

Accepted Article
Accepted Article
Title: Azatruxene-Based, Dumbbell-Shaped, Donor-π-Bridge-Donor
Hole-Transporting Materials for Perovskite Solar Cells
Authors: Luis A. Illicachi, Javier Urieta-Mora, Joaquín Calbo, Juan
Aragó, Cansu Igci, Inés García-Benito, Cristina Momblona,
Braulio Insuasty, Alejandro Ortiz, Cristina Roldán-Carmona,
Agustín Molina-Ontoria, Enrique Ortí, Nazario Martin, and
Mohammad Khaja Nazeeruddin
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.202002115
Link to VoR: https://doi.org/10.1002/chem.202002115
0
1/2020

Chemistry - A European Journal
10.1002/chem.202002115
1
Azatruxene-Based, Dumbbell-Shaped, Donor-
p
-Bridge-
Donor Hole-Transporting Materials for Perovskite Solar Cells
[
a]
[b,c]
[d]
[d]
[e]
Luis A. Illicachi, Javier Urieta-Mora, Joaquín Calbo, Juan Aragó, Cansu Igci, Inés
García-Benito,[ Cristina Momblona, Braulio Insuasty, Alejandro Ortiz, Cristina
Roldán-Carmona, Agustín Molina-Ontoria,* Enrique Ortí,* Nazario Martín,*
Mohammad Khaja Nazeeruddin*[
e]
[e]
[a]
[a]
[
e]
[b]
[d]
[b,c]
and
e]
[
a]
Center for Research and Innovation in Bioinformatics and Photonics-CIBioFi, Grupo de Investigación de
Compuestos Heterocíclicos, Universidad del Valle, Calle 13 No. 100-00, Edificio E20, Cali, Colombia
[
b]
Departamento Química Orgánica, Facultad C. C. Químicas, Universidad Complutense de Madrid, Av.
Complutense s/n, 28040 Madrid, Spain. Homepage: http://www.nazariomartingroup.com/. E-mail:
nazmar@ucm.es
[
c]
IMDEA-Nanociencia, C/ Faraday 9, Ciudad Universitaria de Cantoblanco, 28049 Madrid, Spain. E-mail:
nazmar@ucm.es, E-mail: agustin.molina@imdea.org
[
d]
Instituto de Ciencia Molecular, Universidad de Valencia, Catedrático José Beltrán 2, 46980 Paterna, Spain.
E-mail: enrique.orti@uv.es
[
e]
Group for Molecular Engineering of Functional Materials, EPFL VALAIS, CH-1951 Sion, Switzerland. E-
mail: mdkhaja.nazeeruddin@epfl.ch
Abstract
Three novel donor-p-bridge-donor (D-p-D) hole-transporting materials (HTMs) featuring
triazatruxene electron-donating units bridged by different 3,4-ethylenedioxythiophene (EDOT) p-
conjugated linkers have been synthesized, characterized, and implemented in mesoporous perovskite
solar cells (PSCs). The opto-electronic properties of the new dumbbell-shaped derivatives (DTTXs)
are highly influenced by the chemical structure of the EDOT-based linker. Red-shifted absorption
and emission and a stronger donor ability were observed in passing from DTTX-1 to DTTX-2 due
to the extended p-conjugation. DTTX-3 featured an intramolecular charge transfer between the
external triazatruxene units and the azomethine-EDOT central scaffold, resulting in a more
pronounced red shift. The three new derivatives have been tested in combination with the state-of-
the-art triple-cation perovskite [(FAPbI
PSCs. Remarkable power conversion efficiencies of 17.48 % and 18.30 % were measured for DTTX-
and DTTX-2, respectively, close to that measured for the benchmarking HTM spiro-OMeTAD
3
)
0.87(MAPbBr
3
)
0.13
]
0.92[CsPbI
3
]0.08 in standard mesoporous
1
‒
2
(
18.92 %), under 100 mA cm AM1.5G solar illumination. PSCs with DTTX-3 reached a PCE value
of 12.68 %, which is attributed to the poorer film-formation in comparison to DTTX-1 and DTTX-
. These PCE values are in perfect agreement with the conductivity and hole mobility values
2
determined for the new compounds and spiro-OMeTAD. Steady-state photoluminescence further
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Chemistry - A European Journal
10.1002/chem.202002115
2
confirm the potential of DTTX-1 and DTTX-2 for hole-transport applications as an alternative to
spiro-OMeTAD.
INTRODUCTION
The humankind activities depend to a large extent on the abundance of natural energy sources. Since
the industrial revolution, carbon-based fossil fuels have undoubtedly been our main energy source.
Nevertheless, environmental pollution and global warming raised from the uncontrolled burning of
fossil fuels are causing irreversible harm in biological diversity, the environment, and sustainability
for our society. In this regard, photovoltaic technologies have become one of the most promising
alternatives for harvesting energy from a green and practically endless source. In this context, hybrid
solar cells based on organic-inorganic metal halide perovskites, ABX (A = small organic cation, such
3
+
+
2+
−
−
as methylammonium (MA ) or formamidinium (FA ); B = Pb ; X = Br or I ), have experimented
extraordinary progress in a short time, from the power conversion efficiency (PCE) of 3.8% reported
[
1]
[2]
in 2009 to the most recent 25.2% certified by the NREL. Motivated by the outstanding intrinsic
properties of perovskites (high charge-carrier mobilities, tunable bandgap energy, long free-charge
[
3–7]
carrier diffusion lengths, wide light absorption, and solution-processed fabrication),
the scientific
community has made great efforts to tune accurately these properties using different compositionally
[
8–11]
engineered approaches
including the combination of double or triple cations and different
[
12]
halides. The ambipolar behavior exhibited by perovskites allows their combination in conventional
n-i-p or inverted p-i-n architectures, in which the perovskite is sandwiched between n-type and p-type
[
13]
charge selective layers. To date, the most featured and efficient devices are based on mesoporous
structures, where the perovskite is infiltrated in a mesoporous scaffold of TiO (mp-TiO ) and covered
2
2
with an organic hole-transporting material (HTM). Among the HTM family of interlayers, 2,2´,7,7´-
tetrakis(N,N´-di-p-methoxyphenylamine)-9,9´-spirobifluorene (spiro-OMeTAD) is highlighted as the
[
14,15]
most efficient HTM in combination with different perovskite materials.
Notwithstanding its great
performance, the preparation of the spirobifluorene central scaffold present in spiro-OMeTAD
requires tedious synthetic and purification steps, which significantly limits its practical compatibility
[
15]
for low-cost and large-scale photovoltaic applications.
These drawbacks have motivated the
[
16]
scientific community to design new HTM alternatives, from inorganic materials such as CuSCN,
[
17]
[18]
[19,20]
CuO, or CuGaO
2
,
to organic polymers,
for overcoming the photovoltaic performance of
spiro-OMeTAD. Nevertheless, the most common strategy is based on the use of small organic
molecules, since the chemical versatility of organic compounds allows a precise tune of the
[
21]
optoelectronic properties of the HTM. A wide variety of organic HTMs combining aromatic cores
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Chemistry - A European Journal
10.1002/chem.202002115
3
[
22–24]
[25-35]
such as spiro-based structures
or heteroaromatic structures
with different donor units have
been reported during the last years.
Since the first reports by Bergman and Eklun in 1980, triazatruxene (TAT) has become one of the
most featured heteroaromatic scaffolds, which consists of three indole units fused to a central benzene
ring in a C
3
-symmetry planar fashion. The π-extended conjugated structure of TAT provides high
hole mobility owing to its good electron-donating properties, that facilitates hole extraction, and to
[
36]
its strong π‒π intermolecular interactions, that enhance hole transport. The introduction of alkyl
chains attached to the nitrogen atoms provides excellent solubility to the planar TAT structure and,
[
37]
simultaneously, protect the perovskite layer from the moisture due to their hydrophobic nature.
Taking advantage of these properties, TAT is among the most suitable building blocks for organic
electronic applications, namely organic field-effect transistors (OFETs), organic photovoltaics
(OPVs) or organic light-emitting diodes (OLEDs) due its chemical versatility, which allows easy
[
38][39][40]
modification of its optical and electronic properties.
Star-shaped triazatruxene derivatives
have indeed been employed as HTMs for perovskite solar cells (PSCs) reaching remarkable PCEs up
[
25,41–44]
to 20.3%.
Inspired by the promising features of triazatruxene, we present herein three new dumbbell-shape,
donor-p-bridge-donor (D-p-D) triazatruxene derivatives (DTTX-1, -2 and -3), with different π-
conjugated central cores, as HTMs for PSCs (Figure 1). The three HTMs are prepared following a
simple synthetic strategy, where the central 3,4-ethylenedioxythiophene (EDOT) scaffold is
covalently linked to end-capping TAT units to ensure a suitable highest-occupied molecular orbital
(
HOMO) energy as well as excellent solubility in organic solvents. The new dumbbell-shape
materials have been incorporated in mesoporous PSCs in combination with the triple-cation
perovskite [(FAPbI 0.08 displaying PCEs of 17.42% for DTTX-1,
8.30% for DTTX-2, and 12.68% for DTTX-3. Likewise, a detailed study of the optical,
3
)
0.87(MAPbBr
3
)
0.13
]
0.92[CsPbI ]
3
1
electrochemical, and thermal properties of the new DTTX derivatives, supported with density
functional theory (DFT) calculations, is also provided. The synthesized HTMs show remarkable
PCEs that are comparable to those measured for the state-of-the-art spiro-OMeTAD, thus confirming
the potential of these dumbbell-shape HTMs for highly efficient PSCs.
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Chemistry - A European Journal
10.1002/chem.202002115
4
OMe
OMe
O
O
C6H13
N
MeO
N
N
N
N
OMe
OMe
C6H13
N
N
S
MeO
C6H13
N
C6H13
N
C6H13
N
C6H13
OMe
OMe
DTTX-1
spiro-OMeTAD
O
O
S
O
O
C6H13
N
S
S
C6H13
N
C6H13
N
N
N
C H
6
13
N
S
C6H13
N
N
N
C6H13
N
C6H13
N
C6H13
C6H13
N
C6H13
N
C6H13
DTTX-3
N
DTTX-2
C6H13
Figure 1. Chemical structure of spiro-OMeTAD and the new DTTX-1, DTTX-2, and DTTX-3
HTMs.
RESULTS AND DISCUSSION
Synthesis of DTTX-based HTMs
The synthetic methodology for the preparation of the new triazatruxene derivatives along with the
experimental details are given in the Supporting Information. The TAT units were synthesized
[
25],[45]
following well-established procedures (Scheme S1).
Thus, N-alkylation of triazatruxene (1) and
further controlled bromination afforded monobrominated derivative 3, which was used to prepare the
TAT-boronic acid derivative 4 by standard Miyaura borylation reaction, and the TAT-amine
derivative 5 through Buchwald-Hartwig cross-coupling reaction with benzophenone imine and
subsequently deprotection in acid media. Compound 4 was covalently linked to brominated EDOT
(7) and π-extended EDOT (9) (Scheme S2) through Suzuki cross-coupling reaction to obtain DTTX-
1
and DTTX-2 in high yields (84 and 68%, respectively) (Scheme S3). On the other hand, TAT-
amine derivative 5 was reacted with formyl-EDOT (10) in a low-cost condensation reaction to form
DTTX-3 in high yield (77%). Complete structural characterization of the aforementioned HTMs was
1
13
accomplished using conventional spectroscopic techniques such as H and C NMR, FTIR, and UV–
Vis. High-resolution MALDI-TOF (HR-MS) analysis confirmed the presence of the new DTTX-1‒
3
(for more details, see Supporting Information).
Structural and electronic properties
To gain insight into the structural and electronic properties of the DTTX-1, DTTX-2, and DTTX-3
HTMs, DFT calculations were carried out at the B3LYP/6-31G** level including solvent effects
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Chemistry - A European Journal
10.1002/chem.202002115
5
(
CH
2
Cl ). The hexyl chains attached to the nitrogen atoms of the TAT units were substituted by
2
methyl groups to reduce the number of atoms and thereby the computational cost. The pendant TAT
moiety, the respective EDOT-based cores, and the reference spiro-OMeTAD compound were also
calculated for comparison purposes (see the Supporting Information for full computational details).
Figure 2a displays the minimum-energy geometry calculated for the representative DTTX-2
derivative at the B3LYP/6-31G** level (see Figure S1 in the Supporting Information for all the DTTX
derivatives). The optimized bond lengths computed for the EDOT-based linkers of the three DTTX
compounds are given in Figure S2 for both the DTTX HTMs and the isolated linkers. The thiophene
[
46]
core exhibits the typical structural pattern of related (oligo)thiophene derivatives, with alternating
single and double carbon-carbon bonds forming a π-conjugated pathway. When moving from the
isolated core to the DTTX HTMs bearing the electron-donor TAT units, an enhanced π-delocalization
is predicted as indicated by the decrease in the bond length alternation (average difference in length
between adjacent single C−C and double C=C bonds). For example, the C−C single bond of the
EDOT moiety shortens from 1.440 to 1.431 Å and the C=C double bonds lengthen from 1.366 to
1.379 Å in going from the isolated EDOT linker to the π-extended DTTX-1 (Figure S2a). Similarly,
moving from DTTX-1 to the more conjugated DTTX-2 and DTTX-3, an increase in the C=C double
bonds length and a decrease in the C−C single bonds length of the EDOT-based core support a more
effective π-delocalization upon extending the central linker. The TAT units present slightly twisted
geometries with out-of-plane distortions due to the steric interactions between the N-methyl groups
and the truxene core (Figure 2a and S1). In the fused DTTX systems, a good p-communication of the
constituting TAT−EDOT linker−TAT units is, therefore, anticipated due to the quasi-planar, highly-
conjugated molecular structure, in contrast to that commonly found in similar HTMs decorated with
[
47],[27],[35]
diphenylamine (DPA) and triphenylamine (TPA) peripheral units.
Figure 2b displays the frontier molecular orbitals calculated for the DTTX HTMs, their constituting
units, and the spiro-OMeTAD reference compound (see Table S1 in the Supporting Information for
orbital energies). The HOMO of the EDOT moiety (core1: ‒5.92 eV) increases in energy upon p-
extension with imine groups (core3: ‒5.82 eV) and especially with thiophene rings forming a
terthiophene moiety (core2: ‒5.02 eV). The TAT HOMO is located even higher in energy at ‒4.97
eV. Due to the efficient p-conjugation between the TAT units and the EDOT-type linker, the HOMO
in the DTTX HTMs results from a combination of the HOMOs of the three constituting fragments
(Figure 2b). Noteworthy, the HOMO of DTTX-1 (‒4.64 eV) and DTTX-2 (‒4.60 eV) are only
slightly below the HOMO of spiro-OMeTAD (‒4.44 eV), supporting them as promising HTMs. The
HOMO of DTTX-3 is however more localized over the TAT units and is calculated lower in energy
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Chemistry - A European Journal
10.1002/chem.202002115
6
(‒4.79 eV), leading to a slightly poorer alignment with respect to spiro-OMeTAD (Figure 2b).
Otherwise, the lowest-unoccupied molecular orbital (LUMO) of the EDOT-type linker decreases in
energy upon extension of the p-conjugation (from ‒0.19 eV in core1 to ‒1.65 in core2) and the
incorporation of the electron-withdrawing imine groups in core3 (‒1.78 eV), and from ‒1.46 eV in
DTTX-1 to ‒1.90 and ‒2.25 eV in DTTX-2 and DTTX-3. As a result, the HOMO−LUMO gap
decreases from 3.18 eV in DTTX-1 to 2.70 and 2.54 eV in DTTX-2 and DTTX-3, respectively. In
contrast to the HOMO, the participation of TAT units in the LUMO of DTTX derivatives is marginal,
especially for DTTX-2 and DTTX-3 (Figure 2b).
Figure 2. a) Top (left) and side (right) views of the minimum-energy optimized geometry calculated
for DTTX-2. b) Energy diagram displaying the frontier molecular orbitals computed for the DTTX
HTMs, their constituting molecular fragments, and the spiro-OMeTAD reference compound at the
B3LYP/6-31G** level in CH
2
Cl . For core1, the practically isoenergetic HOMO and HOMO-1 are
2
displayed.
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Chemistry - A European Journal
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7
Optical and electrochemical properties
The optical properties of the new dumbbell-shape HTMs were determined by recording the UV-vis
absorption and fluorescence emission spectra in dichloromethane solution, as outlined in Figure 3a.
The new TAT-based HTMs show strong absorptions in the ultraviolet region with maximum
absorption around 315 nm, which is surmised from the p→p* transitions of the triazatruxene
[
48]
5
‒1
moiety, and present alternative strong bands in the visible centered at 416 nm (e = 1.3 ´ 10 M
‒
1
5
‒1
‒1
cm ) for DTTX-1 and 458 nm (e = 0.9 ´ 10 M cm ) for DTTX-2 due to the p-extended
conjugation with the EDOT central unit and the thiophene linkers. In the case of DTTX-3, a broad
5
‒1
‒1
absorption is observed at 497 nm (e = 0.4 ´ 10 M cm ), which is in principle attributed to an
intramolecular charge transfer (ICT) between the donor triazatruxene units and the acceptor
azomethine-EDOT central scaffold. The emission spectra of DTTX-1 and DTTX-2 present
structured bands with large Stoke shifts and maximum emission centered at 469 and 526 nm,
respectively, which shifts up to 668 nm for DTTX-3. Maximum absorption and emission wavelengths
are collected in Table 1.
Theoretical calculations were performed at the time-dependent DFT-B3LYP/6-31G** level to shed
light on the nature of the electronic transitions that give rise to the experimental absorption bands.
The theoretical absorption spectra (Figure S4 in the Supporting Information) simulated from the
vertical excitation energies and oscillator strengths calculated for the 30 lowest-energy singlet excited
states nicely reproduce the experimental data (Figure 3a). In the 300‒325 nm region, a set of p→p*
electronic transitions mostly involving the TAT units are computed (Table S2 and Figure S4 in the
Supporting Information) that support the experimental peak centered at 315 nm. The intense
absorption band experimentally recorded in the 400‒500 nm region is due to the transition to the
lowest-lying singlet excited state S predicted with a large oscillator strength (f = 1.88, 2.83, and 1.73
1
for DTTX-1, DTTX-2, and DTTX-3, respectively). This transition is described by the
HOMO→LUMO monoexcitation and experiences a redshift in going from DTTX-1 to DTTX-2 and
DTTX-3 in good agreement with the experiment. Note that the red-shift and broader character of this
low-lying band in the case of DTTX-3 compared to the other DTTX derivatives stems from the partial
CT character of the S state. As suggested above and indicated by the topology of the frontier
1
molecular orbitals involved in the transition (HOMO with large TAT participation and LUMO
centered over the thiophene core; Figure 2b), an ICT takes place from the outer TAT units to the
central azomethine-EDOT core.
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Chemistry - A European Journal
10.1002/chem.202002115
8
a)
b)
DTTX-1
DTTX-2
DTTX-3
1.2
1.0
0.8
0.6
0.4
0.2
0.0
1.2
1.0
0.8
0.6
0.4
0.2
0.0
spiro-OMeTAD
DTTX-1
DTTX-2
DTTX-3
300
400
500
600
700
0.0
0.4
0.8
1.2
1.6
Wavelength (nm)
Voltage / V, vs NHE
Figure 3. a) UV-vis absorption spectra (solid line) and fluorescence emission (dashed line) of DTTX-
, DTTX-2, and DTTX-3 recorded in CH Cl . b) Cyclic voltammetry of DTTX-1, DTTX-2, and
DTTX-3 in CH Cl containing 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF ) at a scan
1
2
2
2
2
6
‒
1
rate of 100 mV s .
The redox properties of the TAT-based HTMs were investigated using cyclic voltammetry (CV) in
‒
1
TBAPF
6
/CH
2
Cl at a scan rate of 100 mV s (Figure 3b). For comparison purposes, spiro-OMeTAD
2
was also measured under the same experimental conditions. The oxidation potentials and the derived
HOMO energies of the DTTX derivatives are summarized in Table 1. The cyclic voltammograms of
ox
DTTX-1 and DTTX-2 show multiple oxidation waves with the first oxidation potential (E 1/2) at
0.73 and 0.69 V vs NHE, respectively, leading to HOMO energies of ‒5.17 and ‒5.13 eV for DTTX-
1
and DTTX-2, which are very close to those measured for reference spiro-OMeTAD (‒5.16 eV).
Both new HTMs present a stronger electron-donating ability in comparison to DTTX-3, whose first
oxidation potential (0.84 V) is anodically shifted by 0.11‒0.15 V, resulting in a lower-lying HOMO
at ‒5.28 eV. Hence, an efficient hole extraction from the perovskite to the DTTXs is ensured due to
the adequate band alignment with the valence band edge of the perovskite (ca. ‒5.7 eV). LUMO
energies were experimentally estimated taking into account the intersection between the absorption
and emission spectra (Table 1). Both HOMO and LUMO trends fully agree with the theoretical data
predicted for the frontier molecular orbitals (Figure 2b and Table S1).
To get a more in-depth understanding of the redox properties of HTMs DTTX-1‒3, the Mulliken
atomic charges accumulated by the constituting molecular fragments during the oxidation processes
(neutral → cation and cation → dication) and the theoretical ionization energies associated to those
processes were obtained (Table S3 and S4 in the Supporting Information). Theoretical calculations
indicate that in the first oxidation of DTTX-1 and DTTX-3 the electron is extracted mainly from the
TAT units (0.79 and 0.85e, respectively). In contrast, the first oxidation of DTTX-2 takes place in
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Chemistry - A European Journal
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9
both the terthiophene core and the TAT units (0.5e from the core and 0.5e from the TAT moieties,
Table S3). The first ionization energies are calculated to be 4.50, 4.47, and 4.69 eV for DTTX-1,
DTTX-2, and DTTX-3, respectively, in good accord with the trends in the HOMO energies and the
experimental oxidation potentials, and thus supporting the stronger electron-donating ability for
DTTX-1 and DTTX-2 compared to DTTX-3. For DTTX-1 and DTTX-2, As inferred from Table
S4, the difference between the first and second ionization energies is quite large for DTTX-1 (0.53
eV) and DTTX-2 (0.49 eV), and the first two oxidation waves in the CV experiments are well
separated (Figure 3b). In contrast, this difference is found relatively small for DTTX-3 (0.26 eV),
which could explain the broader and less-defined shape recorded for its first oxidation wave.
Table 1. Optical and electrochemical properties of DTTX derivatives.
ox
0-0
E
[
1/2
a]
E
[eV]
HOMO
[b]
l
[nm]
max, abs
[c]
l
[nm]
max, em
[c]
E
E
[eV]
LUMO
[e]
HTM
[
[d]
V]
[eV]
DTTX-1
DTTX-2
DTTX-3
0.73
0.69
0.84
‒5.17
416
469
2.77
‒2.40
‒5.13
‒5.28
458
497
526
668
2.66
2.37
‒2.47
‒2.91
[
a] Determined from CV measurements vs normal hydrogen electrode (NHE) in CH
eV by EHOMO = ‒4.44 eV ‒ Eox1/2. [c] λmax of absorption and emission was measured in CH
estimated at the intersection of normalized absorption and emission spectra. [e] ELUMO was estimated by ELUMO = EHOMO
2
Cl
2
solution. [b] EHOMO is estimated in
Cl
2
solutions. [d] E0-0 was
2
+
E0-0
.
1
10
00
a)
b)
DTTX-1
DTTX-2
DTTX-3
174 °C
spiro-OMeTAD
DTTX-1
1
131 °C
247 °C
90
80
70
60
50
7
4 °C
210 °C
DTTX-2
78 °C
DTTX-3
132 °C
100
200
300
400
500
600
50
100
150
200
250
300
Temperature (°C)
Temperature (°C)
Figure 4. a) Thermogravimetric analysis (TGA) curves recorded for DTTX-1, DTTX-2, and DTTX-
‒
1
3
under nitrogen atmosphere at 10 °C min of heating rate. b) Differential Scanning Calorimetry
(DSC) curves registered for DTTX-1, DTTX-2, DTTX-3, and spiro-OMeTAD under nitrogen at a
‒
1
heating rate of 20 °C min (first cycle).
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Chemistry - A European Journal
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10
Thermal properties
The thermal properties of DTTX-1, DTTX-2, and DTTX-3 were measured by thermogravimetric
analysis (TGA) and differential scanning calorimetry (DSC) (Figure 4). Detailed thermal features are
collected in Table 2. The new HTMs exhibit good thermal stability with decomposition temperatures
(T
dec, 5% weight loss under N atmosphere) of up to 427 °C (Figure 4a). A noticeable change in
2
thermal stability arises as a result of the chemical structure of the molecular bridging moieties.
Specifically, the azomethine-based DTTX-3 exhibits nearly 60 °C lower thermal stability than the
all-aromatic, backbone-based DTTX-1 and DTTX-2. DSC sheds light on the distinct morphological
behavior of the new compounds. During the first scan, DTTX-1 exhibits a low glass transition (T
g
)
at 74 °C, followed by melting temperature (T ) of 210 °C. No crystallization process was observed
m
upon cooling. During the second cycle, three consecutive processes were observed: a glass transition
at 73 °C, followed by crystallization at 138 °C, and finally a slightly shifted melting temperature of
213 °C (Figure S13 in the Supporting Information). DTTX-2 is essentially amorphous and only a low
glass transition at 78 °C is detected. DTTX-3 exhibits an endothermic peak during the first cycle,
which corresponds to a melting process at 132 °C, without a visible glass transition during the
consecutive cycles.
Table 2. Thermal properties of DTTX-based HTMs.
[
a]
[b]
[b]
HTM
Tdec [°C]
T
m
[°C]
T
g
[°C]
DTTX-1
427
420
373
449
210
‒
74
78
‒
DTTX-2
DTTX-3
132
245
spiro-OMeTAD
125
[
a] Decomposition temperature determined from TGA (5% weight loss). [b] Melting temperature (T
m
) and glass transition
atmosphere.
temperature (T ) determined from the first cycle of the DSC. All experiments were performed under N
g
2
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Chemistry - A European Journal
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11
a)
ELUMO
-
2.0
3.0
4.0
5.0
6.0
-
2.11
-
2.40
-
2.47
-
-
-
-
-2.91
-5.28
TiO2
-4.10
-
4.10
EHOMO
Au
Triple Cation
Perovskite
-
5.16
-5.17
-5.13
-5.10
-5.70
Spiro-
OMeTAD
DTTX-1 DTTX-2
DTTX-3
b)
c)
d)
Au
Au
Au
HTM
HTM
HTM
Perovskite
Perovskite
Perovskite
m-TiO2 / Perovskite
FTO / c-TiO2
m-TiO / Perovskite
2
m-TiO2 / Perovskite
FTO / c-TiO2
FTO / c-TiO
2
1
μm
1 μm
1 μm
Figure 5. a) Energy diagram of the different components used in the PSC. Cross-section SEM
pictures of the corresponding mesoporous (FTO/c-TiO /mp-
TiO /[(FAPbI 0.08/HTM/Au) PSC devices fabricated with b) DTTX-1,
2
2
3
)
0.87(MAPbBr
3
)
0.13
]
0.92[CsPbI ]
3
c) DTTX-2, and d) DTTX-3 as HTM.
Photovoltaic properties.
To evaluate the photovoltaic performance of the dumbbell-shaped DTTX derivatives as HTMs,
solution-processed PSCs were prepared and compared to those built up with the widely used reference
compound spiro-OMeTAD. The photovoltaic devices were fabricated in an n−i−p configuration with
the architecture FTO/c-TiO
state-of-the-art triple-cation perovskite [(FAPbI
harvesting material. The HTMs were applied by spin-coating a chlorobenzene solution (20 mM)
chemically doped with tris(2-(1H-pyrazol-1-yl)-4-tert-butylpyridine)cobalt(III)
2
(30 nm)/mp-TiO
2
(200 nm)/Perovskite/HTM/Au (80 nm) and using the
3
)
0.87(MAPbBr
3
)
0.13
]
0.92[CsPbI ]
3
0.08 as the active light-
tri[bis(trifluoromethane)sulfonimide (FK-209), lithium bis(trifluoromethanesulfonyl)imide (Li-
TFSI), and tert-butylpyridine (t-BP) as additives. Detailed device fabrication is described in the
Supporting Information. The schematic illustration of the energy levels of the different components
of the PSC is displayed in Figure 5a. The new HTMs feature a reasonable band alignment between
their HOMO energy level and the valence band edge of the triple-cation perovskite. Therefore,
effective hole-extraction and efficient electron-blocking are expected for all the HTMs. An optimized
layer of the new doped-HTMs was uniformly spin-coated on top of the perovskite layer with a
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Chemistry - A European Journal
10.1002/chem.202002115
12
thickness of around 80‒100 nm. All derivatives exhibit good solubility in chlorobenzene, which is
favorable for obtaining films showing good morphology and uniform surface coverage of the
perovskite. The cross-section scanning electron microscopy (SEM) images show the individual
constituting layers of the overall device, as displayed in Figure 5b-d. Despite the excellent solubility
showed by DTTX-3, a poorer film formation with the presence of pinholes was observed in
comparison to DTTX-1 and DTTX-2, which has a strong impact on the photovoltaic performance.
a) 25
b) 100
80
25
20
15
10
5
c)
Perovskite
spiro-OMeTAD
DTTX-1
DTTX-2
DTTX-3
20
15
10
5
0
60
40
spiro-OMeTAD
DTTX-1
DTTX-2
spiro-OMeTAD
DTTX-1
DTTX-2
20
DTTX-3
DTTX-3
0
0.0
0.2
0.4
0.6
0.8
1.0
400
500
600
700
800
750
800
850
900
Voltage (V)
Wavelength (nm)
Wavelength (nm)
Figure 6. a) Current density–voltage (J−V) curves of DTTX-1, DTTX-2, DTTX-3, and spiro-
OMeTAD in mesoscopic [(FAPbI 0.08-based devices under typical
solar illumination (1 sun, AM 1.5 G). b) External quantum efficiency (EQE) plots of DTTX-1, DTTX-
, and DTTX-3 with corresponding integrated photocurrent density. c) Steady-state photoluminescence
PL) spectra of pristine triple-cation perovskite and perovskite/HTM films.
3
)
0.87(MAPbBr
3
)
0.13
]
0.92[CsPbI ]
3
2
(
The photovoltaic performance of the aforementioned HTMs in solution-processed PSCs was
‒
2
investigated under simulated illumination of 1 sun AM 1.5 G (100 mW cm ). The current density–
voltage (J−V) curves recorded under reverse bias for the best-performing PSCs are shown in Figure
6a and the values of the main photovoltaic parameters are given in Table 3. DTTX-2-based devices
showed the highest power conversion efficiency (PCE) of 18.30 % with an open-circuit voltage (Voc
)
‒
2
of 1.06 V, a short-circuit current (Jsc) of 23.19 mA cm , and a fill factor (FF) of 0.74 with reduced
hysteresis (Figure S5 in the Supporting Information). Similarly, DTTX-1 devices exhibit slightly
lower efficiencies, the champion device yielding a maximum PCE of 17.42 % (Voc = 1.04 V, Jsc
=
‒
2
2
3.08 mA cm , FF = 0.73) when measuring in the reverse scan direction (see Supporting Information
for a forward scan). In contrast, devices containing azomethine-based DTTX-3 reached a remarkably
lower efficiency, with an optimized PCE of 12.68 %, due to the significantly decreased Voc (0.99 V),
‒
2
J
sc (22.89 mA cm ) and mainly FF (0.56). The reason for the low FF obtained from the latter is most
[
49]
likely due to the high series resistance (R
s
) and low shunt resistance (Rsh
)
, which is attributed to an
[
50]
enhanced interfacial charge recombination. Besides, the low homogeneity and uniformity of the
DTTX-3 film on top of the perovskite layer negatively affects the device performance. The devices
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Chemistry - A European Journal
10.1002/chem.202002115
13
using spiro-OMeTAD as HTM surpassed the PCE values provided by the DTTX derivatives, reaching
a maximum PCE of 18.92 % associated with the enhanced values of Voc (1.08 V) and Jsc (23.44 mA
‒
2
cm ). The calculated Jsc values integrated from the external quantum efficiencies (EQEs) are
consistent with those experimentally observed in all cases (Figure 6b). The long-term stability of
PSCs based on the DTTX HTMs was inspected from unencapsulated devices, which were stored in
ambient air with a relative humidity of 30% ± 5% (Figure S10). DTTX-1 and DTTX-2 exhibited a
similar stability to that found for the spiro-OMeTAD, retaining more than 90% of their initial
efficiency.
To investigate the charge transport and hole collection properties of DTTX-1, DTTX-2 and DTTX-
3
and spiro-OMeTAD, the conductivity was measured by using a two-contact electrical conductivity
set-up with organic field effect transistor (OFET) substrates. All substrates were prepared by spin
coating with the same concentration of dopants as in the fabrication of the devices. Lateral thin-film
-
4
-4
conductivity of doped DTTX-1, DTTX-2, and DTTX-3 was determined to 1.17 × 10 , 4.43 × 10
-
5
-1
and 4.71 × 10 S cm , respectively (Table S6 and Figure S11 in the Supporting Information). The
-
4
-1
conductivity of doped spiro-OMeTAD was determined to be σ = 5.89 x 10 S cm , which is
[
51]
comparable to the values reported in the literature. Although the conductivity values of DTTX-1
and DTTX-2 are slightly lower than spiro-OMeTAD, the conductivity of the layer is enough for
efficient charge transport. However, the conductivity of DTTX-3 is one order of magnitude lower
than the spiro-OMeTAD value, which explains the lower PCE of the device. J-V curves of hole-only
devices of the new HTMs and spiro-OMeTAD were measured in the dark, and the hole mobility (µ
h
)
was determined using the space-charge limited current method (SCLC) (Table S7 and Figure S12 in
[
52]
the Supporting Information). The hole mobility values of doped DTTX-1, DTTX-2, DTTX-3 and
-
5
-4
-5
-4
2
-1
spiro-OMeTAD were determined to be 9.41 × 10 , 1.84 × 10 , 6.70 × 10 , and 3.71 × 10 cm V
-
1
s , respectively, following the same tendency as the lateral conductivity.
The suitable band alignment of the HOMO energy levels of the HTMs and the valence band of the
perovskite is a requirement for efficient extraction of the photogenerated holes at the perovskite/HTM
interface. To evaluate the hole-extraction capability of the DTTX family and spiro-OMeTAD, steady-
state photoluminescence (PL) measurements were recorded. Upon excitation, the pristine triple-
cation perovskite film exhibits a strong PL signal centered at 785 nm (Figure 6c). After capping the
perovskite layer with the different HTMs and spiro-OMeTAD, a decreased PL intensity is observed,
which indicates that the photogenerated holes are efficiently extracted at the perovskite/HTM
interface. The distribution plots of the photovoltaic parameters for 10 devices incorporating the
DTTX HTMs (Figure S6 to S9) indicated a good reproducibility of the different parameters in the
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Chemistry - A European Journal
10.1002/chem.202002115
14
case of DTTX-1 and DTTX-2 in comparison to DTTX-3, which can be attributed to the low
homogeneity exhibited by the DTTX-3 films.
Hole extraction and charge recombination are significantly affected by the morphology of the HTM
films deposited on top of the perovskite layer. To shed light about the surface morphology of spin-
coated films of DTTX-1, DTTX-2, and DTTX-3 onto the perovskite, scanning electron microscopy
was performed (Figure 7). The low-magnification top-view SEM images show the quality difference
between the DTTX films deposited on top of the perovskite layer. DTTX-1 and DTTX-2 layers
present an homogeneous and complete surface coverage of the perovskite. However, the surface of
the DTTX-3 layer displays cases of aggregation of the material after spin-coating deposition, which
is detrimental for the charge extraction and charge transport properties and, therefore, for the device
[
53,54]
performance.
a)
b)
c)
5
0 μm
50 μm
50 μm
Figure 7. Top-view SEM images of a) DTTX-1, b) DTTX-2, and c) DTTX-3 deposited on top of
the perovskite film.
Table 3. Device performance of the champion solar cells containing the new DTTX-1, DTTX-2, and
DTTX-3 HTMs compared with the reference spiro-OMeTAD.
‒
2
HTM
DTTX-1
Voc [V] Jsc [mA cm ] FF PCE [%]
1.04
23.08
23.19
22.89
23.44
0.73
0.74
0.56
0.75
17.42
18.30
12.68
18.92
DTTX-2
1.06
0.99
1.08
DTTX-3
spiro-OMeTAD
CONCLUSIONS
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Chemistry - A European Journal
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15
We have designed, synthesized, and studied the properties of three new donor-p-bridge-donor (D-p-
D) HTMs (DTTX-1‒3) featuring triazatruxenes as end-capping, electron donor units, covalently
bridged by p-conjugated EDOT-based linkers. The azomethine-based DTTX-3 displayed less
thermal stability and start decomposing at significantly lower temperatures (373 °C) in comparison
with its C−C-bonded analogs DTTX-1 and DTTX-2. Increasing the p conjugation of the bridge
resulted in a bathochromic shift of 42 nm for absorption and 57 nm for emission in passing from
DTTX-1 to DTTX-2. The azomethine-EDOT linker narrows in a higher degree the HOMO−LUMO
energy gap and produces an additional red shift of both absorption and emission. The mesoporous
[
(FAPbI
3
)
0.87(MAPbBr
3
)
0.13
]
0.92[CsPbI ]0.08-based PSCs using DTTX-3 as HTM reached a maximum
3
PCE of 12.68%, which is attributed to the low uniformity of the spin-coated layer that enhances the
interfacial charge recombination. Under the same conditions, DTTX-1 and DTTX-2 yielded PCEs
up to 17.42% and 18.30%, respectively, which are comparable to the performance achieved with
spiro-OMeTAD (18.92%), becoming promising candidates for replacement of spiro-OMeTAD in
highly efficient PSCs.
ACKNOWLEDGEMENTS
We thank the Swiss National Funds for Scientific Research contract number 200020L_172929/1. The
authors also thank the Spanish Ministry of Science, Innovation and Universities MCIU (projects
CTQ2017-83531-R, PGC2018-099568-B-I00, RED2018-102815-T, Centro de Excelencia Severo
Ochoa SEV-2016-0686 and Unidad de Excelencia María de Maeztu MDM-2015-0538), the CAM
(QUIMTRONIC-CM
project
Y2018/NMT-4783),
the
Generalitat
Valenciana
(PROMETEO/2016/135 and SEJI/2018/035) and European Feder funds (PGC2018-099568-B-I00).
J.A. is grateful to MCIU for a “Ramon-y-Cajal” fellowship (RyC-2017-23500). L.I. thanks to the
Universidad del Valle (CIAM-2017) and the Science, Technology and Innovation Fund-General
Royalties System (FCTeI-SGR) under contract BPIN 2013000100007 for a predoctoral fellowship.
Borun New Material Technology generously supplied the high-quality spiro-OMeTAD.
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Chemistry - A European Journal
10.1002/chem.202002115
19
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TOC
C6H13
N
O
O
C6H13
N
N
S
C6H13
N
C6H13
N
C6H13
N
C6H13
DTTX-1
h = 17.42%
Metal contact
HTM
O
O
S
S
S
C6H13
Perovskite
N
C6H13
N
N
C6H13
N
N
C6H13
C6H13
m-TiO
2
N
C6H13
DTTX-2
h = 18.30%
c-TiO
2
C6H13
N
O
O
C6H13
N
N
N
N
S
FTO
C6H13
N
C6H13
N
C6H13
N C6H13
DTTX-3
h = 12.68%
Donor-p-Bridge-Donor triads based on triazatruxene units covalently linked through different p-
conjugated spacers, in a dumbbell-shape fashion, have been synthetized and tested in perovskite
solar cells as hole-transporting materials. Power conversion efficiencies up to 18.3% in conventional
mesoporous perovskite solar cells have been achieved, showing a strong dependence on the surface
morphology of the new hole transporting materials.
This article is protected by copyright. All rights reserved.